1. Field of the Invention
The present invention relates to a measurement apparatus which measures the distance between a reference surface and a surface to be measured.
2. Description of the Related Art
A light wave interference measurement apparatus using a multi-wavelength light source is known as an apparatus which measures the distance between a reference surface and a surface to be measured (see Japanese Patent Laid-Open No. 62-135703 and Japanese Patent No. 2808136). This measurement apparatus needs to selectively increase the number of wavelengths to widen the length measurement range. For example, to obtain a length measurement range of about 1 m with a near infrared light source, four wavelengths need to be used. Japanese Patent No. 2808136 proposes a technique of decreasing the number of light sources by changing the wavelength by current modulation of an optical communication semiconductor laser in order to suppress complication and cost rise of the measurement apparatus caused by the increase in the number of wavelengths. Also, a wavelength scanning light wave interference measurement apparatus which continuously scans the wavelength of a light source is known as an apparatus which measures the distance between a reference surface and a surface to be measured (see Japanese Patent Publication No. 06-052166).
In this measurement apparatus, the guaranteed accuracy of the wavelength of the light source is a factor of the length measurement accuracy (measurement accuracy). To measure the distance at high accuracy, the wavelength of the light source needs to be guaranteed at high accuracy. From this, the techniques in Japanese Patent No. 2808136 and Japanese Patent Publication No. 06-052166 guarantee the wavelength of the light source using a wavelength reference such as an etalon or gas cell (that is, the guaranteed accuracy of the wavelength of the light source is increased).
However, in wavelength guarantee using an etalon disclosed in Japanese Patent No. 2808136, the transmittance peak of the etalon is sensitive to an environmental change. Thus, it is difficult to guarantee the wavelength at high accuracy for a long period. Also, in wavelength guarantee using a gas cell disclosed in Japanese Patent No. 2808136, the minimum wavelength interval of the light source cannot be set equal to or smaller than the absorption line interval of gas. It is therefore difficult to satisfactorily ensure the length measurement range (that is, measure the distance in a wide range). For example, the absorption line interval of acetylene gas used as a wavelength reference in the optical communication band is about 500 pm. In this case, the distance can be measured only within the range of about 2.4 mm.
The wavelength scanning measurement apparatus disclosed in Japanese Patent Publication No. 06-052166 needs to increase the wavelength scanning amount in order to measure the distance at high accuracy. This measurement apparatus cannot use a low-cost light source such as a semiconductor laser, raising the cost of the light source.